I just purchased a 20G long and I want to use an Aqua Clear filter. It's going to be a planted tank. I don't want too big a filter that will blow the c02 out. The 20 (Aqua Clear) looked too small--would a thiry be o.k. It' not going to be heavily stocked with fish. A school of white clouds and a few shrimp and about 3 cory cats. I plan to use excel, not dosing c02. HELP!